Dog Crossing

Five people (A, B, C, D and Lisa) and
five dogs (a, b, c, d, and Lisa's dog)
are on a hiking trip. Each person owns one dog (the letters match). They
come to a river and want to cross it. A rubber raft will hold up to three
at a time, any combination of dogs and humans.
Only the five people and Lisa's dog can drive the raft.
How do you get them across the river?
The catch: a dog cannot be in the
presence of any humans unless its owner is also there.
[Addendum: assume everyone exits the raft after each trip.]

Source: A tutored student of
John Knoderer's of
www.MAZES.com
on newsgroup rec.puzzles 9/14/97. I was later told it is related to an old
Henry Dudeny puzzle.